Add Superscript and Subscript font styles

[s-johri]

This adds superscript and subscript text support.

pdf.SetFont("font", "", 14)
pdf.Cell(nil, "E = mc")
pdf.SetFontWithStyle("font", gopdf.Superscript, 14)
pdf.Cell(nil, "2")
pdf.SetFontWithStyle("font", gopdf.Regular, 14)
pdf.Cell(nil, ", said Einstein")

How it works

• Two new style bits, Superscript = 8 and Subscript = 16. Like Underline, they are decoration bits: they reuse the already-loaded font face (the face lookup now strips all decoration bits, not just Underline).
• The glyph size and baseline shift are derived from the font's own OS/2 metrics (ySuperscriptYSize/YOffset, ySubscriptYSize/YOffset), which ParseOS2 previously skipped over. Fonts with missing or zero metrics fall back to conventional ratios (58.3% size, +33.3%/-14.1% baseline shift).
• Rendering uses the PDF text-rise operator: the state line becomes /F1 9.1 Tf 0 Tc 6.34 Ts. Ts is emitted on every draw (0 Ts for plain text) because text state persists across BT/ET blocks, so each draw must reset it.
• The caller's font size keeps governing vertical layout (line height, baseline placement), so script runs share the baseline of surrounding text with no caller arithmetic. Width measurement (MeasureTextWidth, wrapping, justify adjustments) uses the scaled size so measurements match rendering.

Notes for review

• The string style API ("B"/"I"/"U") is not extended: getConvertedStyle matches single letters via Contains, so "SUP"/"SUB" would falsely match Bold/Underline. The flags are int-style only.
• Underline rectangles are intentionally not shifted by the rise; that can follow if anyone needs underlined superscripts.
• TestSuperscriptSubscriptVisualDemo writes test/out/superscript_subscript_demo.pdf for visual checking (chemical formulas, exponents, footnote markers).

[oneplus1000]

Hello again! Thank you for PR. I will check and review it on the weekend.